Monostable controlled rectifier switching circuit with variable impedance for low power dissipation and rapid recovery



July 29, 1969 R. L. GAMBLIN ,7 MQNOSTABLE CONTROLLED RECTIFIER SWITCHING CIRCUIT WITH VARIABLE IMPEDANCE FOR LOW POWER DISSIPATION AND RAPID RECOVERY Filed June 5, 1966 //Vl E/VTO/? RODGER L. GAMBLIN ATTORNEY United States Patent MONOSTABLE CONTROLLED RECTIFIER SWITCH- ING CIRCUIT WITH VARIABLE IMPEDANCE FGR LOW POWER DISSIPATION AND RAPID RECOVERY Rodger L. Gamblin, Vestal, N.Y., assignor to International Business Machines Corporation, Armonk, N.Y., a corporation of New York Filed June 3, 1966, Ser. No. 555,196 Int. Cl. H03k 3/26 U.S. Cl. 307-273 5 Claims This invention relates generally to an improved circuit for driving resistive and inductive loads at relatively high current levels and which is particularly well adapted to driving electromechanical transducers such as the coils which are utilized to drive print hammers in impact printing apparatus.

Impact printing operations involve the movement of a mechanical element which contains a relatively high energy level. This movement is impulsive in nature, that is, it must be accelerated to a relatively high speed in a very short time. When this motion is impelled by an electromechanical transducer, the nature of the operation demands that high electrical power levels be employed in driving the transducer during the period of acceleration.

The most common approach in high speed impact printers and the like is the use of a power transistor for energizing each hammer coil together with one or more preamplifiers which provide suflicient input drive to the power transistor. Although this type of drive circuit performs well at high speeds, the power transistor, together with the preamplifier stages, results in an overall high cost for driving each hammer.

Within recent years, silicon-controlled rectifiers have been employed more and more in environments wherein they replace power transistors. The silicon-controlled rectifiers are particularly advantageous because of significant advantages in both cost and size. In addition, relatively low input power levels can switch high output power levels, thereby eliminating the need for poweringup stages. With the advent of monolithic technology and the adaptability of the silicon-controlled rectifier to fabrication in this new technology, the size and cost factors are even more enhanced.

Accordingly, it is a primary object of the present invention to provide an improved print hammer (or other load) drive circuit utilizing silicon-controlled rectifiers.

The use of silicon-controlled rectifiers in the hammer drive circuitry is not new. Several circuit arrangements have been suggested, some of which make use of the rectifiers in a bistable mode of operation and in a few instances, in a monostable mode of operation. For various reasons relating to dual input control and timing problems, the use of the monostable mode of operation becomes particularly advantageous. Known silicon-controlled rectifier driver circuits of the monostable type have suffered from various timing and control problems, particularly those related to fast recovery times which are necessitated in high speed printers. Practical embodiments have also suffered from excessive power dissipation.

It is therefore a more specific object of the present invention to provide an improved, high speed, monostable device utilizing silicon-controlled rectifiers.

These objects are achieved in a preferred embodiment of the invention by providing a first silicon-controlled rectifier connected in series with the load impedance between the terminals of a suitable power supply. A second silicon-controlled rectifier and a series impedance are connected across the power supply. The anodes of the two rectifiers are coupled to each other through a 3,458,730- Patented July 29, 1969 "ice capacitor, the function of which is to turn off an energized rectifier when the other rectifier is initially turned on. The impedance which is in series with the second rectifier is in the form of a transistor which provides a very high impedance when the circuit is in its inactive state and which provides a very low impedance for charging the capacitor during the cyclical energization of the monostable device. This variable impedance significantly improves the recovery time to permit high speed operation, almost eliminates power loss in the stable stat of the circuit, and assures greater reliability of operation. The conductivity of the second rectifier is controlled by means of a second capacitor and its related charge and discharge circuits.

It is therefore a more specific object of the present invention to provide, in a monostable device comprising a pair of silicon-controlled rectifiers and a coupling capacitor, a nonlinear impedance which provides fast recovery time for the circuit, thereby to significantly improve the maximum speed of operation, to provide for an elimination of power wastage in the stable state, and to improve the reliability of the circuit.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawing which is a schematic diagram of the improved monostable device.

The improved monostable device includes a first siliconcontrolled rectifier 1 which is connected in series with a load impedance 2 between ground potential and a positive supply terminal 3.

A second silicon-controlled rectifier 4, a transistor 5 and an emitter resistor 6 are connected in series between ground potential and the supply terminal 3.

The anodes of the rectifiers 1 and 4 are connected to each other by way of a coupling capacitor 7. The base electrode of the transistor 5 is connected to the anode of the rectifier 1 by way of a current limiting resistor 8.

The anode of the rectifier 4 is connected to ground potential by way of a voltage divider comprising resistors 10 and 11. The junction between the latter resistors is connected to ground potential by way of a diode 12 and a capacitor 13. The junction between the diode and capacitor is connected directly to the control electrode of the rectifier 4 and is also connected to the supply terminal 3 by way of a potentiometer 14 and a resistor 15. The control electrode 16 of the rectifier 1 is connected to a source of turn-on pulses (not shown).

The operation of the monostable device is as follows. The rectifier 1 is initially in its nonconductive state, whereby the positive potential at the terminal 3 is applied to the right-hand terminal of the capacitor 7.

A conductive path is established by way of resistors 11, 14 and 15 and diode 12. The value of the resistor 11 is so small relative to the value of resistors 14 and 15 that there is substantially no voltage drop across the resistor 11. The junction between the diode 12, capacitor 13 and the control electrode 4 will, therefore, be fixed at approximately a positive seven-tenths volt level (e.g. the diode drop), whereby the rectifier 4 is forward biased to its low impedance state. However, since its anode is substantially at ground potential, the anode to cathode current flow is insignificantly small. The left-hand terminal of the capacitor 7 is, therefore, at ground potential.

The transistor 5 is normally in its non-conducting state since the positive potential from the terminal 3 is applied to both its base and emitter electrodes.

When a positive pulse is applied to the input terminal 16, the rectifier 1 is turned on to energize the load 2. At the same time, the capacitor 7 applies a negative pulse to the junction between the transistor 5 and the rectifier 4,

driving this junction to a highly negative level. This negative potential turns off the rectifier 4 and causes the capacitor 13 to be charged to a predetermined negative potential determined 'by the relative values of the resistors 10 and 11. The capacitor 13 reverse biases the control electrode of the rectifier 4.

The transistor 5 is now forward biased and begins to conduct. Due to its low impedance, the transistor 5 will very rapidly charge the capacitor 7 substantially to the voltage difference between ground potential and the potential level at the terminal 3. During this time interval, the capacitor 13 will be charging positively by means of the charge circuit formed by the resistors 14 and 15, diode 12 being reverse biased.

When the charge across the capacitor becomes sufficiently positive to forward bias the junction between the control electrode and the cathode of the rectifier 4, the latter rectifier is turned on. This causes the capacitor 7 to apply a highly negative potential to the anode of the rectifier 1, turning the latter rectifier off.

The capacitor 7 is charged by way of the load impedance 2; and, when the voltage level at the anode of the rectifier 1 reaches a level substantially equal to that at the supply terminal 3, transistor 5 is turned off. With the transistor 5 nonconducting, the anode circuit of the rectifier 4 is interrupted except for the small current flow through the resistors 10 and 11; and the circuit adjusts itself to the initial state which existed before the rectifier 1 was turned on.

Suitable component values which provide satisfactory operation are given by way of example below; however, the scope of the application is not to be limited thereby:

Resistors:

6 values in ohms 28 8 do 370 10 do 1,200 11 do 210 15 do 72,000 14 do 20,000

Capacitors:

7 values, microfarads 4.1 13 do .051

It will be appreciated that the improved results obtained by the use of the nonlinear impedance, i.e. transistor 5, can be utilized in a bistable or other form of the circuit illustrated. For example, the resistors 10, 11, 14 and 15, the diode 12 and the capacitor 13 can be removed; and rectifier 4 can be turned on when desired by an external pulse source.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A circuit for energizing a load with high energy pulses of short time duration of the type:

in which first and second silicon-controlled rectifiers are connected for sequential energization to momentarily energize the load over a series path including the first rectifier, and

in which means, including a capacitor coupled to similar electrodes of the rectifiers, is effective to apply a turn-off pulse to each rectifier incident to the initial turn-on of the other rectifier,

the circuit being characterized by:

a variable impedance element in series with the second rectifier and connected for operation in its high impedance state while the first rectifier is nonconducting to minimize power drain through the second rectifier, and for operation in its low impedance state while the first rectifier is conducting for rapidly conditioning the capacitor for subsequent turning off of the latter rectifier.

2. The circuit set forth in claim 1 wherein the variable impedance element comprises:

a transistor switch having its base-emitter electrodes connected in series with the first rectifier and having its collector electrode connected to the second rectifier and the capacitor.

3. A monostable circuit comprising:

a power supply having first and second terminals,

a first silicon-controlled rectifier and a load impedance element connected in series between the terminals,

a second silicon-controlled rectifier and a variable impedance element connected in series between the terminals,

a capacitor interconnecting the junctions between the rectifiers and their respective impedance elements,

first means normally maintaining each rectifier in a substantially nonconductive state,

the first rectifier adapted to respond to input signals for switching to a highly conductive state to energize the load element,

second means for initiating the switching of the second rectifier to a highly conductive state at a predetermined time interval after it is rendered effective;

third means including the capacitor responsive to switching of the first rectifier for rendering the latter means effective,

means causing the variable impedance element to switch from a high to a low value in response to switching of the first rectifier to its highly conductive state and rapidly charge the capacitor to a desired value prior to switching of the second rectifier to its highly conductive state,

said capacitor responsive to the switching of the second rectifier to its highly conductive state for applying a turn-off pulse to the first rectifier; and

said variable impedance thereafter switching to a high value to inhibit significant current flow through the second rectifier.

4. The monostable circuit set forth in claim 3 wherein the variable impedance comprises:

a transistor switch having its base-emitter electrodes connected in series with the first rectifier between the supply terminals and having its collector electrode connected to the second rectifier and the capacitor.

5. The monostable ci-rcuit set forth in claim 4:

wherein the second means comprises an integrating circuit including a second capacitor and resistance means, and

wherein the third means includes a voltage divider connected to the first-mentioned capacitor and a diode connecting an intermediate junction of the voltage divider to the second capacitor.

References Cited UNITED STATES PATENTS 3,193,733 7/1965 Orsino 307252 3,231,812 1/1966 Paley 307252 3,238,418 3/1966 Heft 307-252 ARTHUR GAUSS, Primary Examiner R. H. PLOTKIN, Assistant Examiner US. Cl. X.R. 

1. A CIRCUIT FOR ENERGIZING A LOAD WITH HIGH ENERGY PULSES OF SHORT TIME DURATION OF THE TYPE: IN WHICH FIRST AND SECOND SILICON-CONTROLLED RECTIFIERS ARE CONNECTED FOR SEQUENTIAL ENERGIZATION TO MOMENTARILY ENERGIZE THE LOAD OVER A SERIES PATH INCLUDING THE FIRST RECTIFIER, AND IN WHICH MEANS, INCLUDING A CAPACITOR COUPLED TO SIMILAR ELECTRODES OF THE RECTIFIERS, IS EFFECTIVE TO APPLY A TURN-OFF PULSE TO EACH RECTIFIER INCIDENT TO THE INTIAL TURN-ON OF THE OTHER RECTIFIER, THE CIRCUIT BEING CHARACTERIZED BY: A VARIABLE IMPEDANCE ELEMENT IN SERIES WITH THE SECOND RECTIFIER AND CONNECTED FOR OPERATION IN ITS HIGH IMPEDANCE STATE WHILE THE FIRST RECTIFIER IS NONCONDUCTING TO MINIMIZE POWER DRAIN THROUGH THE SECOND RECTIFIER, AND FOR OPERATION IN ITS LOW IMPEDANCE STATE WHILE THE FIRST RECTIFIER IS CONDUCTING FOR RAPIDLY CONDITIONING THE CAPACITOR FOR SUBSEQUENT TURNING OFF OF THE LATTER RECTIFIER. 